1. Field of the Invention
The present invention relates to an electric power steering control device and, in particular, to an electric power steering control device for controlling an electric power steering mounted on an automobile or the like.
2. Description of the Related Art
An electric power steering is widely used in recent years because of advantages such as improvement of stability during a hand-free operation and lightness of steering secured during an operation of a steering wheel. As indicated by an example shown in FIG. 20, an electric power steering is a device for detecting a steering torque and using assist map control to supplement a steering torque of a driver by an electric motor in accordance with a predetermined target assist torque. Assist map control is a control system for using a steering torque of a driver as an input to supplement a necessary torque according to a magnitude of the steering torque. As a conventional electric power steering, there is one constituted by a damping compensator mainly for securing stability during a hand-free operation and an inertia compensator mainly for preventing steering from becoming heavy affected by inertia at the time of swift steering to secure lightness of steering. Damping compensation is a control for using a speed of a motor as an input to supplement a steering torque of an operator according to a magnitude of a motor speed and has an effect of stabilizing a vehicle. In addition, inertia compensation is a control for using an acceleration of a motor as an input to supplement a torque equivalent to inertia imposed on an operator in starting to turn a steering wheel or turning the steering wheel in the other direction and has an effect of compensating heavy steering even at the time of swift steering and always securing lightness of steering.
A conventional control device for controlling an electric power steering of this type is proposed, for example, in JP 11-139339 A. In this conventional control device, a steering torque of an operator is supplemented based on a vehicle speed signal value and a measured or estimated motor speed value and motor acceleration value.
In the conventional control device, since an amount of control based on a motor speed and a motor acceleration is kept constant, steering may become heavier than needed if the operator steers fast. In addition, a damping gain and an inertia compensation gain are constant. Thus, in order to increase an amount of damping compensation, since it becomes a resistance (steering shaft reaction) when the operator steers fast, it is necessary to change an assist map.
As described above, in the conventional control device, since an amount of control based on a motor speed and a motor acceleration is kept constant, steering becomes heavier than needed if an operator steers fast.
In addition, as described above, in the conventional control device, a damping gain and an inertia compensation gain are constant. Thus, in order to increase an amount of damping compensation, since it become a resistance when the operator steers fast, it is necessary to change an assist map for compensation of damping.
The present invention has been devised to solve the above-mentioned problems, and it is an object of the present invention to provide an electric power steering control device that prevents steering from becoming heavier than needed even if a steering wheel is steered fast and makes it unnecessary to change an assist map.
The electric power steering control device of the present invention for controlling the electric power steering, comprises: the vehicle speed detecting means for detecting a vehicle speed; the steering torque detecting means for detecting a steering torque of the steering wheel; the steering shaft reaction torque outputting means for finding and outputting a steering shaft reaction torque; the motor speed detecting means for detecting a motor speed; the base target current calculating means for calculating a base target current for driving the motor based on the vehicle speed and the steering torque; the damping gain calculating means for finding a gain of a damping torque based on the steering shaft reaction torque; the damping compensation amount calculating means for calculating a damping compensation amount based on the gain of the damping torque and the motor speed; and the compensating means for compensating the base target current by the damping compensation amount to calculate a target current. Therefore, it is possible to prevent the steering from becoming heavier than needed even when the steering wheel is steered fast and to make it unnecessary to change the assist map.